In recent years, OCT that forms images of the surface morphology and internal morphology of an object by using a light beam from a laser light source or the like has attracted attention. Unlike an X-ray CT apparatus, OCT is noninvasive to human bodies, and is therefore expected to be utilized in the medical field and biological field. For example, in the ophthalmology, apparatuses that form images of a fundus and a cornea are in a practical stage.
Patent Document 1 discloses a device to which OCT is applied. This device has such a configuration that: a measuring arm scans an object by a rotary deflection mirror (a Galvano mirror); a reference arm is provided with a reference mirror; and an interferometer is mounted at the outlet to analyze, by a spectrometer, the intensity of an interference light of light fluxes from the measurement arm and the reference arm. Moreover, the reference arm is configured to gradually change the light flux phase of the reference light by discontinuous values.
The device of Patent Document 1 uses a technique of so-called “Fourier Domain OCT.” That is to say, the device irradiates a low coherence light beam to an object, superposes the reflected light and the reference light to generate an interference light, and acquires the spectral intensity distribution of the interference light to execute Fourier transform, thereby imaging the morphology in the depth direction (the z-direction) of the object. The technique of this type is also called Spectral Domain.
Furthermore, the device described in Patent Document 1 is provided with a Galvano mirror that scans with a light beam (a signal light), and is thereby configured to form an image of a desired measurement target region of the object. Because this device is configured to scan with the light beam only in one direction (the x-direction) orthogonal to the z-direction, an image formed by this device is a two-dimensional tomographic image in the depth direction (the z-direction) along the scanning direction (the x-direction) of the light beam.
Patent Document 2 discloses a technique of scanning with a signal light in the horizontal direction (x-direction) and the vertical direction (y-direction) to form a plurality of two-dimensional tomographic images in the horizontal direction, and acquiring and imaging three-dimensional tomographic information of a measured range based on the tomographic images. As the three-dimensional imaging, for example, a method of arranging and displaying a plurality of tomographic images in the vertical direction (referred to as stack data or the like), and a method of executing a rendering process on a plurality of tomographic images to form a three-dimensional image are considered.
Patent Documents 3 and 4 disclose other types of OCT devices. Patent Document 3 describes an OCT device that images the morphology of an object by sweeping the wavelength of light that is irradiated to an object, acquiring the spectral intensity distribution based on an interference light obtained by superposing the reflected lights of the light of the respective wavelengths on the reference light, and executing Fourier transform. Such an OCT device is called a Swept Source type or the like. The Swept Source type is a kind of the Fourier Domain type.
Further, Patent Document 4 describes an OCT device that irradiates a light having a predetermined beam diameter to an object and analyzes the components of an interference light obtained by superposing the reflected light and the reference light, thereby forming an image of the object in a cross-section orthogonal to the travelling direction of the light. Such an OCT device is called a full-field type, en-face type or the like.
Patent Document 5 discloses a configuration obtained by applying OCT in the ophthalmic field. The device described in this document includes a function that captures an image of a fundus to form a fundus image, and a function that measures the fundus using OCT to form tomographic images and three-dimensional images (collectively referred to as “OCT images”). Furthermore, this device analyzes tomographic images and identifies image regions corresponding to layer structures configuring the fundus. The layer structures subject to identification include the inner limiting membrane, the nerve fiber layer, the ganglionic cell lamina, the inner plexiform layer, the inner granular layer, the external plexiform lamina, the external granular layer, the external limiting membrane, the photoreceptor cell layer, and the pigment layer of the retina, etc. Furthermore, because the fundus is configured by layering multiple layer structures, obtaining image regions corresponding to layer structures is equivalent to obtaining image regions corresponding to the boundary positions of adjacent layer structures. Furthermore, from before the use of OCT, retinal cameras have been widely used as devices for observing the fundus (refer to, for example, Patent Document 6).
A device using OCT is advantageous compared to a retinal camera with respect to the fact that it is capable of acquiring high-definition images, and is also capable of acquiring tomographic images and three-dimensional images.
In recent years, there has been growing attention on the ocular disease known as age-related macular degeneration. Age-related macular degeneration is a disorder that occurs due to age-related decreases in function in the macula area of the retina, and causes symptoms such as distortions in visual range, decreases in eyesight, difficulty seeing parts of one's field of view, and an inability to observe a target despite being able to observe surrounding areas normally.
Age-related macular degeneration (exudative) is believed to occur through the following mechanism. Normal retinal cells undergo repeated regeneration. Under normal conditions, waste matter generated during regeneration is dissolved within the retinal pigment epithelium and disappears. However, if the actions of the retinal pigment epithelium decrease due to aging, undissolved waste matter accumulates between the Brusch's membrane and the pigment layer of the retina. When a fundus in this state is imaged, the waste matter is observed as white clusters known as drusen. When the waste matter accumulates, a weak inflammatory response occurs. When this happens, specific chemical substances (chemical mediators) are produced to promote the healing of the inflammation. However, the chemical mediators include agents that promote the generation of blood vessels, and as a result, new blood vessels are generated from the choroid. If the new blood vessels burst through the Brusch's membrane and penetrate to the area below or above the pigment layer of the retina and proliferate, the exudation of blood and blood components becomes acute and decreases in function of the macula become severe.
The presence and distribution of drusen are important factors for diagnosing age-related macular degeneration. Conventionally, fundus images (captured images from a retinal camera) have mainly been used to understand the state of drusen (refer to, for example, Patent Document 7).